Gaseous etching of molybdenum



US. Cl. 156-18 United States Patent 3,518,134 GASEOUS ETCHING OFMOLYBDENUM Ruth C. Preist, Menlo Park, Calif, assignor to StanfordResearch Institute, Menlo Park, Calir., a corporation of California NoDrawing. Filed Aug. 14, 1967, Ser. No. 660,241 Int. Cl. C23b 3/00; C23f1/02 11 Claims ABSTRACT OF THE DISCLOSURE A procedure for the precisionetching of molybdenum surfaces at an etch rate of 0.1 to 1 micron ormore per minute with a high etch factor is disclosed by heating thesurface to a temperature above 300 C. while delivering to the surface amixture of oxygen and hydrogen chloride gases at a pressure below aboutone atmosphere and removing the volatile oxychloride reaction gases fromthe etched surface. Precision etched patterns are obtained by the use ofa thin film of aluminum oxide resist film on the surface.

BACKGROUND OF THE INVENTION Field of the invention The present inventionrelates to a method of etching molybdenum and more particularly to theprecision high etching of heated thin films of molybdenum with a mixtureof gases.

Description of the prior art Reactions leading to a solution orvolatilization of molybdenum may not be acceptable as etchants forseveral reasons. An etchant should provide good dimensional control andleave a smooth, undamaged finish surface. The reaction, even ifsufficiently vigorous, may leave a residue on the surface or mayselectively attack grain boundaries or dislocations leading to unevenetch rates and patterns. The corrosive chemicals may destroy resistlayers or damage adjacent structures or underlying layers. It is furtherdesirable that the etchant be based on lowpressure gases so that theetching steps can be accomplished in the same equipment used forfabrication of thin film devices.

SUMMARY OF THE INVENTION .The etching method of the invention utilizesgaseous reactants at low pressure which react with the surfacemolybdenum film to form a volatile compound which on removal leaves aclean, undamaged surface which is at least a smooth as the pre-etchsurface. The gases do not attack an aluminum oxide resist film andremove molybdenum with good control of depth and degree of undercuttingto provide high resolution and precision etched patterns.

Briefly, in themethod ofthe invention, a mixture of oxygen and hydrogenchloride gases at a low pressure below about atmospheric and preferablybelow 100 torr are delivered to the surface of a molybdenum article at atemperature above about 300 C. and the volatile react ionproducts areremoved. This results in a very favorable etch: rate of 0.01 to 1 micronper minute.

"The smooth removal of molybdenum by oxygen and hydrogen chloride gasesat high rate is quite unexpected sincepure HCl does not etch molybdenumat all under similar conditions and the reaction of molybdenum film withoxygen is very slow and a thick residue remains on the surface. Theremoval of molybdenum according to the invention is believed to beaccompplished by the two- 3,518,134 Patented June 30, 1970 Thetemperature at the surface should be maintained above about 300 C. sincethis is the temperature at which the molybdenum oxychloride becomesvolatile and the reactions shown above -will occur. The uppertemperature of the etching reaction is only limited by decomposition ofthe molybdenum trioxide and the oxychloride. However, acceptable etchrates are obtained without exceeding 800 C. and less likelihood ofdamage to the associated structures and better control are obtainable attemperatures below 600 C.

At the lower temperatures the rate of etching appears to be limited bythe rate of formation of molybdenum trioxide which is observed to bevery slow at 300 C. with diatomic oxygen and quite fast at 600 C.However, it is believed that since energy of dissociation is notrequired and the probability of diffusion is increased, the surfacetemperature can be lowered to the 300 C. to 400 C. range whilemaintaining a reasonable reaction rate. It would also appear desirableto bombard the surface with slow electrons or radio frequency radiantenergy to speed the reaction at the surface.

At temperatures below 770 C. lower oxidation states of molybdenum areformed. A dioxide layer possibly containing some intermediate oxides isfound nearest the metal surface while the trioxide exists at the gasinterface. The MoO is an intermediate in the overall removal ofmolybdenum but if the reaction is allowed to go to completion the filmof M00 is also removed. The rate of removal of molybdenum is dependenton the thickness of the layers and correspondingly on the rates offormation and dissociation of the oxides as well as the stability andrate of diffusion of oxygen through the layers. All of these factors aretemperature dependent.

If all the reaction products were volatile and there was no boundarylayer of molybdenum oxide, it is not probable that the reaction zonewould be dilfusion limited to the surface molecular layer as it is inthe high temperature metal-gas reaction of concern and is believed tocontribute to the even and smooth etched surfaces.

The above postulated mechanism is borne out by the fact that a thin,pale brown film of relatively hard, adherent and transparent materialremains on a partially etched surface. This film has been identified asM00 a compound which is stable to about 1780 C. when heated in bulk invacuum. However, under any fixed set of conditions of gas pressure, gascomposition and substrate temperature, eventually an equilibrium will beestablished between the rate of growth of the oxide film and the rate ofremoval of M00 by formation of MoO Cl Thus, an increase of the totalpressure by increasing the partial pressure of either HCl or 0 decreasesthe M00 film thickness by increasing the probability of molybdenumtrioxide formation and its reaction with HCl, before the M00 gives upits 0 to form M00 in the underlying Mo. Thus the rate of removal ofmolybdenum increases. Lower temperatures also promote lower dioxide filmthickness since the dissociation of the trioxide is less favored.

It has further been discovered that, for a given gas mixture, there is acertain critical temperature where the oxide film appears to have aminimum thickness and the etch rate is at a maximum. This temperature isbelieved to correspond to the temperature at which the reaction,dissociation and diffusion rates are balanced to give the most efficientetching. Variation of the temperature above or below increases thethickness of the film and lowers the etch rate. An increase in oxygenpartial pressure was observed to move this band of maximum etch rate toa region of lower temperature and an increase in HCl rate moved the bandof maximum etch rate to a region of higher temperature. The choice ofconditions depends on whether a thinner oxide film and faster etch rateor thicker oxide film and better etch factor or resolution are desired.

Several alternatives exist for the removal of the dioxide film afteretching. The film can be fired in hydrogen to reduce the oxide tomolybdenum metal. However, the residue of molybdenum metal could beobjectionable because of its texture or structure. Chlorine gas reactswith the substrate at a temperature above 600 C. to rapidly remove theoxide film but it has been found that the chlorine also attacksmolybdenum extremely rapidly under these conditions, and 'would destroyall dimensions established by the original etching.

A suitable method which can be effected in the same vacuum equipmentwithout movement of the substrate is to adjust the substrate temperatureto about 300 C. to about 500 C. and the pressure to above 1 torr,preferably 25 to 100 torr so that surface reaction rates are temporarilyfavored over dissociation and diffusion rates. Again, the use of atomicor ionic oxygen may have a favorable effect by increasing the surfacerate of formation of the oxychloride film.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Illustrative methods of etchingaccording to the invention are offered by way of illustration in thefollowing specific examples which are not intended in any way to belimiting since alternate or modified modes of operation are permissiblewithout deviating from the scope of the invention.

A series of one-half inch squares of 0.030 inch thick bariumborosilicate as drawn glass substrate specimens were cleaned and coatedwith one-half to 1 micron thick films of molybdenum by evaporation invacuum from an electron bombardment heat source. The films were laiddown at rates of about one to four per hour. Some of the molybdenumcoated substrates were further coated with an A1 pattern as resist.Polystyrene balls of onequarter to 1 micron diameter were blown from anebulizer over the molybdenum surface and the A1 0 was deposited overthem by evaporation in vacuum from an electron bombardment source. Theballs were easily removed by a short ultrasonic cleaning in water anddetergent leaving clearly defined holes in the resist slightly largerthan the diameter of the polystyrene spheres.

The coated glass squares were placed in a vacuum bell jar on top of aquartz envelope having a flat top surface and containing a carbonheating filament. The isolation of the radiant heater prevents unwantedreactions be tween the etchant gases on the filament and its supportsand connections. The filament was energized to heat the substrate to atemperature from 400 C. to 700 C. by radiation through the quartz sleeveand oxygen and hydrogen chloride gases were metered into the jar frompressurized tanks and the gases were removed by means of a mechanicalvacuum pump as illustrated in the following examples.

EXAMPLE 1 A sample containing a 4500 A. thick molybdenum film and a 1000A. thick aluminum oxide resist film containing holes formed byone-quarter balls was heated to 670 C. in the reaction chamber and 3.8cc./sec. of oxygen and 0.2 cc./sec. of HCl were delivered to the belljar at a pressure of about 0.3 torr. The reaction products and unreactedgases were continuously removed for a period of 17 minutes. Themolybdenum was etched at a rate of about 0.025 micron per minute.

The molybdenum surfaces exposed through the holes in the resist wereuniformly removed and the etched surface was as smooth or smoother thanthe pre-etched surface. The etch factor which is defined as the ratio ofthe depth of etch divided by the distance etched under the edge of theresist has been at least about two. Dimensional control accurate tobetter than 0.1 micron in films 1 micron thick is permissible and evengreater precision is attainable in films thinner than 1 micron. For agiven set of etching conditions of temperature, time, pressure and gascomposition the rate and condition of the etched surface can beaccurately reproduced.

The minimum amount of HCl is that required to remove the MoO before itcan dissociate and combine with the underlying Mo to form M00 whichremains on the etched surfaces. At the temperatures under considerationand at pressures from 0.1 to 1 torr as little as 1 volume percent HClpresent in the gas mixture appears to provide practical etch rates. Infact, over certain ranges of temperature and pressure the rate ofetching increases with increased pressure provided by increasing theamount of either HCl or 0 admitted to the reaction chamber. This isapparently due to the suppression of dissociation of the trioxide andthe increased probabilities of formation of trioxide and of the volatileoxychloride. This phenomena is illustrated below.

EXAMPLE 2 A sample identical to that of Example 1 was heated to 670 C.in the reaction chamber and 7.8 cc./sec. of O and 0.2 cc./sec. of HClgas were delivered to the bell jar at a pressure of about 0.6 torr andthe reaction products and unreacted gases were continuously removed fora period of ten minutes. The molybdenum was etched at a rate of about0.045 micron per minute.

The 0.1 micron A1 0 resist layer is unaffected by the etchant gases andit has been found that A1 0 resist films at least 0.15 micron thick areimpervious to the etchant gases at temperatures up to 800 C. The surfacecharacteristics and shapes of the holes was about the same in eachexperiment. Other replicate samples were etched at substratetemperatures of 480 C. and 580 C. with flow rates of HCl from 0.1cc./sec. to 0.2 cc./sec. with a reaction chamber pressures from about0.4 to l torr. The molybdenum was etched at a rate of 0.01 to 0.6 micronper minute.

Each molybdenum specimen partially etched at lower pressure containedthe hard, transparent and adherent film of molybdenum dioxide. This filmcan be removed by favoring surface reaction rates over dissociationrates by increasing pressure at lower etch temperature according to thefollowing procedure exemplified with respect to an uncoated specimen.However, the oxide coated samples can be treated in an identical mannerto remove the oxide film.

EXAMPLE 3 A molybdenum coated glass square was placed in the evacuatedvacuum bell jar and heated to 400 C. A premeasured oxygen and HClmixture in a volume ratio of -O /HC1 of 4/1 was introduced to provide apressure of 0.1 atmosphere (76 torr). The molybdenum film was removed atthe rate of one micron per minute.

The rate of formation of the molybdenum oxychloride was suflicientlyhigh in this case to prevent formation of any residual transparentdioxide film which is usually of such thickness to give a browninterference color. The etch rate was simultaneously increased to a rateof 1 micron per minute. Since the final etched surface was still verysmooth, it appears that the reaction is still limited to the surfacemolecular layer even at the higher pressure of this experiment. However,precision of depth of etching is better controlled by use of lowerpressures and corresponding etch rates.

The presence of the oxide layer during etching contributes to thesmoothing action and is important when it is desired to control the etchrate of contoured surfaces. With the oxide layer present, molybdenumarticles are reduced uniformly without preferential attack on certaincrystal faces or attack of grain boundaries. This has been demonstratedby the etching of a length of 1.5 mil diameter wire with a temperaturegradient along its length. The wire was reduced smoothly and uniformlyfrom a cross-section area of 38p. to about 4 Thus, the process of theinvention can be utilized to reduce the size of massive molybdenumarticles, for the precision etching of molybdenum surfaces or selectiveprecision etching of resist coated thin evaporated molybdenum films.Another use of the process is the polishing and more importantly thecleaning of molybdenum film surfaces. Much diificulty is experienced inthe bonding of junctions to molybdenum films or in the lamination ofthese films to other surfaces because of the greasy or oxidizedcondition of the molybdenum surface. However, by means of the gaseousetching process of the invention the surface can be prepared for bondingwith a precise removal of an extremely thin surface film of molybdenum.

The lack of corrosive or selective attack of surfaces, the possibilityof control of the etch factor, uniformity in hole shape and size, andrelatively easy control of etching conditions are further importantcharacteristics of the process of the invention.

It is to be understood that the foregoing only relates to preferredembodiments of the invention and that numerous substitutions,modifications, or alterations of the specifically disclosed materials orprocedures are permissible all without departing from the scope of theinvention as defined in the following claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A method of etching molybdenum articles comprising the steps ofheating the surface of the molybdenum article to a temperature fromabout 300 C. to 800 C.;

etching the surface at a rate of 0.1 to 1 micron per minute bydelivering to the heated surface at a pressure of from about 0.1 to 100torr a mixture of oxygen and hydrogen chloride gases containing at leastone percent by volume of HCl to form volatile by-products; and

removing from the surface the volatile by-products.

2. A method according to claim 1 in which the surface is heated to atemperature of from about 400 C. to 700 C.

3. A method according to claim 1 in which a molybdenum oxide layer isformed on the surface during the etching.

4. A method according to claim 2 in which the molybdenum surface is inthe form of a thin molybdenum film deposited onto a substrate.

5. A method according to claim 4 in which the film is etched to aprecision of 0.1 micron.

6. A method according to claim 3 in which only a portion of thethickness of the molybdenum article is etched and the oxide film is thenremoved.

7. A method according to claim 6 in which the oxide film is removed byreaction with a member selected from the group consisting of chlorinegas, hydrogen gas, and a mixture of hydrogen chloride and oxygen gases.

8. A method according to claim 7 in which the temperature of the film isadjusted to a temperature between 300 C. and 500 C. and the pressure ofthe oxygen and HCl gases is increased to a pressure above 1 torr toremove the molybdenum oxide film.

9 A method according to claim 1 in which the surface of the molybdenumarticle is coated with a layer of a resist pattern and the molybdenum isselectively etched in the discontinuities in the pattern.

10. A method according to claim 9 in which the resist layer is aluminumoxide.

11. A method according to claim 10 in which the aluminum oxide layer isat least 0.1 thick.

References Cited FOREIGN PATENTS 7/ 1965 Germany.

OTHER REFERENCES ROBERT F. BURNETT, Primary Examiner R. J. ROCHE,Assistant Examiner US. Cl. X.R. l48l.5; 156-13

